In EAS systems, significant advantages are obtained using an "active" tag as opposed to the traditional "passive" tag. The "active" tags each include a receiver unit for receiving signals and decoding messages therein, an alarm unit and a signal processor to decode messages for selectively operating the alarm unit to provide sensible output alarm indication. Such "active" tag is disclosed in U.S. Pat. No. 4,686,513 which is incorporated herein by this reference thereto.
The '513 patent describes an application in which multiple transmitters, each transmitting within a confined area, communicate with "active" tags within their boundaries. For example, in a store, a first transmitter may be stationed at the exit area, thereby providing a protected exit area. A tag brought without authorization into the exit are will be interrogated and an appropriate alarm will be initiated. A second transmitter is provided at the checkout counter, therebY providing a protected checkout area. A tag within the checkout area can be instructed to assume a variety of operating states. For example, the tag can be caused to issue an alarm output to prompt checkout clerk removal thereof from a purchased article. Further, the tag can be placed in a "sleep" state, accordingly to pass through the exit area in authorized manner if not removed from the article.
Large installations typically involve a substantially-sized controlled surveillance area and accordingly may require plural active transmitters in addition to the exit and checkout transmitters, which may themselves be in pluralities.
Of primary concern in increasing the number of transmitters is the amount of interference between transmitters which may be mutually unsynchronized. Interference reduces the sensitivity of the system and degrades the performance thereof. Increasing the distance between the unsynchronized transmitters decreases the interference. However, a significant loss in flexibility of installation results when spacing between transmitters is required.
Another consideration of applicants is in addressing generally the synchronizing of a plurality of signal transmission systems. It is typical in prior art signal transmission systems that one identifies a "master" among the plurality of participating systems and identifies all other participating systems as "slaves". The shortcoming of such prior art arrangement is that the overall network is at the mercy of the identified master system continuing operationally and maintaining synchronization of the network. Thus, where the master system becomes inoperative after synchronization, the network fails at large as respects synchronous transmissions.